Take care of last newline while capturing commands output [duplicate] - bash

Question

This question already has answers here:
Capturing multiple line output into a Bash variable
(7 answers)
Closed 6 years ago.
I know $(cmd) captures the output of cmd into a string. However, it doesn't take care of the newline characters at the end.
Here's a demonstration:
a=$(echo x)
b=$(echo -n x)
[ "$a" = "$b" ] && echo equal
a and b capture outputs differed by a "\n" but these variables have the same value.
So my goal is still to capture the output of a command, but this time I want to preserve the last newline character(s) if there are any.

Answer 1

Trailing newlines are removed
POSIX requires that the $(…) notation (or the equivalent using back-ticks) strips all trailing newlines from the end of the string that is captured.
§6.2.3 Command substitution
…, removing sequences of one or more <newline> characters at the end of the substitution.
There isn't a simple way around that, or to detect how many newlines were deleted.
Add a single newline to the end of the output
If you have Bash 4.x (4.3 tested) you can play with shell parameter expansion and the substring notation, and add a dummy line of output to the end of the original string (that's the echo n in this example):
$ x=$(echo pandemonium; blanklines 4; echo n)
$ echo "$x"
pandemonium
n
$ y=${x: 0: -1}
$ echo "$y"
pandemonium
$
When using Bash 3.2 (GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin16)) on macOS Sierra, the expansion for y generates an error:
$ y=${x: 0: -1}
-bash: -1: substring expression < 0
$
See also Capturing multiple line output to a Bash variable.
Reliable and POSIX-compliant
Or you can use a simpler, more portable (POSIX-compatible) substitution suggested by the answer — as applied to this answer:
$ y=${x%n}
$ echo "$y"
pandemonium
$
Given that this works in strict POSIX shells and both Bash 3.x and 4.x, there's no need for the substring variant.